1) Field of the Invention
The present invention relates to dielectric inserts and, more particularly, to dielectric inserts that are verifiable and used for lightning strike protection between a structure and a substructure.
2) Description of Related Art
Competition in the commercial aircraft industry has created demand for higher performance aircraft with lower manufacturing and operating costs. To meet this demand, new materials (such as composite materials) and fabrication processes must be evaluated and applied to new designs. Although composite materials have been used on a number of military and commercial aircraft in non-fuel areas, the use of composite materials for fuel filled (a.k.a., wet) composite primary structure poses a significant lightning strike concern and change in aircraft design philosophy and certification. Lightning protection is, and has been, a certification requirement for over 30 years. The vast majority of legacy commercial aircraft designs have been based on use of aluminum as the primary airframe material. Aluminum is an excellent electrical and thermal conductor and provides a tremendous amount of inherent lightning protection. Relatively inexpensive, low impact design criteria have been successfully applied to aluminum-based platforms to obtain safe structural, fuel tank, and system designs with known certification approaches. Composite structures, which have a diminished capacity for carrying electrical current relative to aluminum, are far more susceptible to damage from lightning strike attachments.
Because composite aerospace structures are susceptible to lightning strike damage, research has been focused on lightning strike protection (LSP). The protection approaches are aimed at either protecting surfaces (especially load-carrying surfaces) from excessive damage or puncture, or aimed at enabling the safe transport of current between attachment points. This is particularly important when currents are conducted through fuel-containing areas like wing fuel tanks. The risk from uncontrolled transport of current is that certain geometries are prone to developing sparks above geometry-specific threshold levels (threshold is defined as the lowest value of the peak current of a transient lightning pulse at which arcing or sparking is seen to occur). One such geometry involves mechanically fastened skin-substructure joints where exposed fastener heads can conduct high currents from the airplane exterior into metallic substructures. For these geometries, the sparking threshold can be quite low (on the order of 5000 amps), due mainly to the interface between fastener collars/nuts and metal surfaces.
To improve sparking in these geometries, it has been determined that by electrically isolating the collars from the metal substructure with a dielectric insert the spark threshold can be raised substantially. Ten times or greater improvement in threshold values have been achieved.
Dielectric inserts have additional requirements to meet in addition to the ability to withstand high electrical currents without breakdown. Some of these requirements include the ability to transfer compressive loads between collars and the substructure, the ability to withstand long-term fuel exposure, and the ability to be inspectable and unable to be inadvertently removed or substituted during collar installation (either during initial assembly or in-service). These requirements are necessary because when an insert is not properly placed between the collar and substructure, protection against arcing/sparking is nonexistent. Similarly, if a non-dielectric insert (e.g., a conductive metallic insert) is inadvertently substituted for the dielectric insert, then lightning strike protection is also circumvented.
Various approaches for development of a dielectric insert meeting the necessary requirements have been considered and developed. These approaches include dielectric washers, insulative coatings, and bonded glass/epoxy layers. These approaches suffer from a number of problems. For example, hard insulative coatings can be scratched or penetrated inadvertently during assembly operations. Dielectric washers suffer from being relatively easy to be substituted for or to be not installed in the first place. In addition, there is the risk of substituting a dielectric washer for a metallic washer during maintenance/repair operations. To avoid the risk of inadvertent non-installation, bonded composite (e.g., glass/epoxy) dielectric inserts have also been considered. However, bonded glass/epoxy lamina inserts require expensive and time consuming bonding operations, and are also somewhat susceptible to damage during assembly, including damage due to collar installation. Their ability to withstand cyclic loading over the lifetime of the aircraft is also unreliable.
With all dielectric insert approaches, convenient/economical verification of the inserts' presence in the structure is highly recommended. The primary problem with all of the previous isolation approaches is that they cannot be verified to be in existence without extremely inconvenient and time consuming manual visual inspections of a structure, such as the inside of the fuel tank. Such inspections can be further hindered by the presence of fuel sealant covering the regions requiring inspection.
It would therefore be advantageous to provide an insert that is capable of being non-destructively located and verified. In addition, it would be advantageous to provide an inspection system that is portable and capable of inspecting structures to verify the presence of dielectric sensors. It would also be advantageous to provide a non-destructive inspection system that is effective and economical to manufacture and use.